public static IEnumerable<T> Distinct<T, V>(this IEnumerable<T> source, Func<T, V> keySelector,
    IEqualityComparer<V> comparer = EqualityComparer<V>.Default)
{
    return source.Distinct(new CommonEqualityComparer<T, V>(keySelector, comparer));
}

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Linq;

public class CommonEqualityComparer<T, V> : IEqualityComparer<T>
{
    private Func<T, V> keySelector;
    private IEqualityComparer<V> comparer;

    public CommonEqualityComparer(Func<T, V> keySelector, IEqualityComparer<V> comparer)
    {
        this.keySelector = keySelector;
        this.comparer = comparer;
    }

    public CommonEqualityComparer(Func<T, V> keySelector)
        : this(keySelector, EqualityComparer<V>.Default)
    {  }

    public bool Equals(T x, T y)
    {
        return comparer.Equals(keySelector(x), keySelector(y));
    }

    public int GetHashCode(T obj)
    {
        return comparer.GetHashCode(keySelector(obj));
    }
}